I'm using a TreeModel to represent a filesystem directory tree for
presentation by a JTree. The first level of the directory appears fine,
but when I click on any of the little triangles to expand a
subdirectory, I get an exception,

apple.awt.EventQueueExceptionHandler Caught Throwable :
java.lang.NullPointerException
       at FileSystemTreeModel.getChild(Unknown Source)

This is on Mac OS X 10.3, JDK 1.4.2.

Here are some of the methods I implemented for my TreeModel. I am
thinking that perhaps I need to do something special when the user
clicks on the triangle? I don't quite understand that ... why wouldn't
the JTree just call back to the appropriate methods of my TreeModel?

Any advice appreciated.

class FileSystemTreeModel implements TreeModel {
   String dir;
       
   public FileSystemTreeModel (String dir) {
       this.dir = dir;
   }

   public Object getRoot() {
       return new File(dir);
   }

   public Object getChild(Object parent, int index) {
       File f = (File)parent;
       if (! f.isDirectory()) {
           return null;
       }
       
       MyFile mf = new MyFile(parent.toString());
       File[] files = mf.listFiles();
       return files[index];
   }
       
   public int getChildCount(Object parent) {    
       File f = (File)parent;      
       
       if (! f.isDirectory()) {
           return 0;
       }
       
       MyFile mf = new MyFile(f.getAbsolutePath());
       File[] files = mf.listFiles();
       return files.length;    
   }
       
   public boolean isLeaf(Object node) {
       File f = (File)node;
       return f.isFile();
   }

       
   public int getIndexOfChild (Object parent, Object child) {
       File f = (File)parent;
     
       if (! f.isDirectory()) {
           return -1;
       }
       
       MyFile mf = new MyFile(f.getAbsolutePath());
       File[] files = mf.listFiles();
       for (int i = 0; i < files.length; ++i) {
           if (files[i].equals(child)) {
               return i;
           }
       }    
       return -1;
   }